The gap in the F21D2 is really a big risk as it seems. Tested against Sub-Zero and he can punish it in all variation with everything but Ice Ball and Aura.
Slide, Ice Burst, Clone, Hammer and Parry all work. Just wait till second hit connects and armor through it :-/ Parry gives him a combo as you get frozen. Hammer and Ice Burst launch you for potential combo follow-up.
Here the break down from my testing against Sub (Sub moves all ex)
Vanish - works for everything. Shortest duration is enough for everything but Hammer. You need to hide a split second longer to dodge Hammer. Sub seems to recover faster from the Ice Burst as Ermac from the Vanish.
Teleport - avoids slide & the Hammer (ex teleport beats hammer and launches Sub). Parry & Clone give him a free combo; Ice Burst launches you and he can basically do whatever he wants.
Levitate - avoids everything but Ice Burst. Havent checked on the follow ups though. Maybe you can combo Sub out of it afterwards... Also prevents the clone from coming out; Sub jumps back but no Clone appears. Sub Zero gets a run cancel combo of off Ice Burst.
Soul Burst/Lift/Push - hits Sub if he goes Clone(?) or Parry. Rest will punish you. Ice Burst might give him a combo. I put a ? on the clone cuz i might be wrong... Clone wont out, Sub just gets hit. Either the clone is to slow to come out in general or i am to slow with the input
Soul Ball - same as push or lift with the difference that it doesnt break Subs parry. Lift and Push on the other hand hit Sub while his parry activates.
So against Unbreakable the risk for this string isnt too high it seems. Against Cryo and Grandmaster it looks extremly risky.
Sry for the wall of text.
P.S. porbably the wrong Thread for this... but didnt know how to put it and since we started this issue here...
EDIT: Dont think its a big changer but... ex Push sometimes beats ex Slide and Sub gets pushed away after loosing the Armor trade.